1. Field of the Invention
The present invention relates generally to circuit protection devices, and particularly to self-testing circuit protection devices.
2. Technical Background
Circuit protection devices are configured to interrupt the flow of electrical power to a load circuit when certain fault conditions. Two of the most common types of circuit protection devices are arc-fault circuit interrupters (AFCIs) and ground-fault circuit interrupters (GFCIs). AFCI and GFCI protection may be included together in one protective device.
An arc fault is a discharge of electricity between two or more conductors. An arc fault may be caused by damaged insulation on the hot or neutral line conductors, or both. The damaged insulation may cause a low power arc between the two conductors and fire may result. An AFCI is configured to detect the arcing condition and de-energize the electrical circuit.
A ground fault occurs when a current carrying (hot) conductor contacts ground. This creates an unintended current path that represents an electrical shock hazard. A ground fault creates an unintended current path that may also lead to fire. GFCIs intended to prevent fire have been called ground-fault equipment protectors (GFEPs.)
A ground fault may occur for several reasons. If the wiring insulation within a piece of equipment becomes damaged, a user may contact the hot conductor and ground at the same time, creating a shock hazard for a user. A ground fault may also occur when the equipment comes in contact with water and the user comes in contact with the water. A ground fault may also be caused by damaged insulation within a structure. A GFCI is configured to sense dangerous conditions such as these and respond quickly. Under normal operating conditions, the current flowing in the hot conductor should equal the current in the neutral conductor. Thus, GFCIs typically compare the current in the hot conductor to the return current in the neutral conductor by sensing the differential current between the two conductors. When a ground fault occurs, the current flowing in the hot conductor does not equal the current in the neutral conductor, differing by the amount of the unintended ground fault current. The GFCI may respond by actuating an alarm and/or interrupting the circuit. Circuit interruption is typically effected by opening the line between the source of power and the load.
Another type of fault may occur when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded. This condition does not represent an immediate shock hazard. Under normal conditions, a GFCI will trip when the differential current is greater than or equal to approximately 6 mA. However, when the load neutral conductor is grounded and the user comes in contact with the hot conductor, only a portion of the fault current through the user is diverted to ground. When this happens, it may take up to 30 mA of fault current through the user to produce 6 mA of differential current before the GFCI trips. Thus, when a double-fault condition occurs, i.e., when both the hot conductor and the load neutral are grounded, the GFCI may fail to trip, causing serious injury or death.
The GFCI includes components that can malfunction, unbeknownst to the user. When an internal fault has occurred, the GFCI may fail to trip during both the grounded hot fault condition and the grounded neutral fault condition.
In light of the above discussion, it is desirable to provide a GFCI that is capable of self-testing to assure that the GFCI is affording protection for both the grounded hot fault condition and the grounded neutral fault condition. In one approach that has been considered, a GFCI has been configured to include a timer that initiates a periodic self test of the GFCI. Alternatively, the GFCI initiates a periodic alarm to alert the user to manually push the test button on the GFCI. One drawback to this approach is that the circuitry is relatively expensive and increases the size of the GFCI circuitry.
In another approach that has been considered, a GFCI includes a visual indicator adapted to display a mis-wire condition. If the hot power source conductor and the neutral power source conductor are inadvertently mis-wired to the load terminals of the GFCI, the visual indicator is actuated to display the mis-wire alarm condition. Those of ordinary skill in the art will understand that a mis-wire condition of this type will result in a loss of GFCI protection at the duplex receptacles on the face of the GFCI. One drawback to this approach is that the GFCI does not include a self-test of the electrical circuit. Another drawback to this approach is that the visual display does not indicate a lock-out of load side power by the interrupting contacts. As such, the user is obliged to correctly interpret and take action based on appearance of the visual indicator.
In yet another approach that has been considered, a GFCI is configured to self-test the relay solenoid that opens the GFCI interrupting contacts when a fault condition is sensed. However, the self-test does not include a test of the electrical circuit.
In yet another approach that has been considered, the self-test is configured to detect the failure of certain components, such as the SCR. If a failure mode is detected, the device is driven to a lock-out mode, such that power is permanently de-coupled from the load.
In light of all of the approaches discussed above, there are many other types of failures, such as those involving the GFCI sensing circuitry, that require manual testing. Of course, manual testing requires a user to push the test button disposed on the GFCI. If a simulated fault condition is present, the GFCI trips out after the test button is pushed. This prompts the user to reset the GFCI. If the device fails to reset, the user understands that the device has failed and is in a lock-out condition. This approach has drawbacks as well. While regular testing is strongly encouraged by device manufacturers, in reality, few users test their GFCIs on a regular basis.
Therefore, there is a need for a GFCI that is configured to self-test the GFCI sensing circuitry. There is a further need for a GFCI that is adapted to self-test for both the grounded hot fault condition and the grounded neutral fault condition. Finally, there is a need for a self-testing GFCI which performs self-testing every half-cycle, during a time period when the SCR tripping mechanism does not conduct.